This invention relates to formulations for cathode materials for use in batteries and more particularly to cathode materials for use in rechargeable Li-ion batteries that are charged to high voltage.
Several failure mechanisms have been reported in Li-ion batteries, which limit their performance in practical applications. One of the most important failure modes is the one that involves the surface of the cathode material. This mode of failure mainly arises from catalyzing effects of the transition metals present in the cathode electrode, which can oxidize the electrolyte. This process, which can happen even at low voltage (<4V), forms an insulating layer on electrodes, limits easy ion transport, and decreases the electronic conductivity. The outcome of this process leads to a rise in the impedance of the cathode, resulting in massive cell failure.
One approach to prevent this phenomenon is to modify the surface of the cathode particles with a thin coating of an oxide compound with preferably high conductivity. An ideal surface-modified cathode material will have good ionic and electronic conductivity, and in addition will have a stable cathode-electrolyte interface. Further, the coated material also acts as a protective layer against acid attacks, such as HF, and minimizes metal ion dissolution. Examples of coatings on LiMn2O4 spinel material include MgO [K. Amine, C. H. Chen, J. Liu, M. Hammond, A. Jansen, D. Dees, I. Bloom, D. Vissers, G. Henriksen, J. Power Sources, 97-98, 684-687 (2001)], Al2O3 [A. M. Kannan, A. Manthiram, Electrochem. & Solid State Lett, 5, A167 (2002); H. Liu, Y. P. Wu, E. Rahm, R. Holze, H. Q. Wu, J. Solid State Electrochem, 8, 450 (2004)], ZrO2 [S. C. Park, Y. M. Kim, Y. M. Kang, K. T. Kim, P. S. Lee, J. Y. Lee, J. Power Sources, 103, 86 (2001)], and ZnO [Y. K. Sun, Y. S. Lee, M. Yoshio, K. Amine, Electrochem. Solid State Lett. 5, A99 (2002)]. These coatings, despite improving the stability and cycle life of the cathode materials, are considered as a non-uniform coating. Additionally, such coatings only partially cover the surface of the cathode particles. This leaves the surface of uncoated cathode particle (“weak spots”) vulnerable to reaction with the electrolyte, which adversely affects the stability of the Li-ion cell. In addition, these coatings are passive (i.e. inactive), and do not change the physical and chemical properties of the core material. The stability issue is exacerbated in cathodes when charged to a high voltage, i.e. >4V. Therefore, to eliminate the drawbacks of the available surface modification procedures, a more effective method is required to enhance the stability of the electrode-electrolyte interface.